regexp_replace weirdness amounts to a bug?

Hello,

The following surprised me enough to think it might be a bug:
(17devel)

select
    regexp_replace('Abc Def'
   , '([a-z]) ([A-Z])'
   , '\1 ' || lower('\2')  );

regexp_replace
----------------
  Abc Def
(1 row)

-- 'Abc Def' got
-- 'Abc def' expected

What do you think?

Thanks,

Erik Rijkers